Integrated photovoltaic-battery device and related methods

ABSTRACT

Provided are FeS 2  based photovoltaic battery devices comprising a transparent substrate, an active layer disposed over the transparent substrate, the active layer comprising a porous film of FeS 2  nanocrystals and a halide ionic liquid infiltrating the porous film, and an electrode disposed over the active layer. The device may be configured such that under exposure to light, photons incident on the active layer are absorbed by the FeS 2  nanocrystals, generating a current and a voltage, whereby a separation of charge within the active layer is created, which is discharged in the absence of the light.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/US2013/076982 that was filed on Dec. 20, 2013, which claims thebenefit of U.S. Provisional Patent Application No. 61/848,118 that wasfiled on Dec. 24, 2012, the entire contents of which are herebyincorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under grant numberCMMI-1332658 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

BACKGROUND

Earth abundant iron pyrite (FeS₂) nanostructured materials have beenproposed in renewable energy applications, such as photovoltaics (PVs),energy storage batteries and photocatalysts. (See Ennaoui, A.;Tributsch, H., Iron sulfide solar-cells. Solar Cells 1984, 13 (2),197-200; Choi, J.-W.; Cheruvally, G.; Ahn, H.-J.; Kim, K.-W.; Ahn,J.-H., Electrochemical characteristics of room temperature Li/FeS₂batteries with natural pyrite cathode. Journal of Power Sources 2006,163 (1), 158-165; Kirkeminde, A.; Ren, S., Thermodynamic control of ironpyrite nanocrystal synthesis with high photoactivity and stability.Journal of Materials Chemistry A 2013, 1 (1), 49-54; Puthussery, J.;Seefeld, S.; Berry, N.; Gibbs, M.; Law, M., Colloidal iron pyrite (FeS₂)nanocrystal inks for thin-film photovoltaics. Journal of the AmericanChemical Society 2011, 133 (4), 716-9.) In addition, due to its hightheoretical capacity (890 mAh/g) and low environmental impact, FeS₂ isan attractive cathode material in Lithium Ion Batteries (LIB). (SeeShao-Horn, Y.; Osmialowski, S.; Horn, Q. C., Nano-FeS₂ for CommercialLi/FeS₂ Primary Batteries. Journal of The Electrochemical Society 2002,149 (11), A1499.) Regarding photovoltaic applications, iron pyrite isattractive due to its high photoabsorption coefficient (above 10⁵ cm⁻¹)and ideal light harvesting bandgap (0.95 eV). (See Puthussery, J.;Seefeld, S.; Berry, N.; Gibbs, M.; Law, M., Colloidal iron pyrite (FeS₂)nanocrystal inks for thin-film photovoltaics. Journal of the AmericanChemical Society 2011, 133 (4), 716-9.) However, the efficiency of FeS₂PV devices has been extremely modest. (See Steinhagen, C.; Harvey, T.B.; Stolle, C. J.; Harris, J.; Korgel, B. A., Pyrite Nanocrystal SolarCells: Promising, or Fool's Gold? The Journal of Physical ChemistryLetters 2012, 3 (17), 2352-2356.) In particular, FeS₂ is a semiconductorwith low conductivity and high surface trap states. (See Birkholz, M.;Fiechter, S.; Hartmann, A.; Tributsch, H., Sulfur deficiency in ironpyrite (FeS_(2-x)) and its consequences for band-structure models.Physical Review B 1991, 43 (14), 11926-11936.) In addition, synthesizedcolloidal FeS₂ nanostructures are typically surrounded with a layer oflong chain organic ligands (e.g., octadecylamine (ODA), oleic acid),which are usually one to several nanometers in length. (See Puthussery,J.; Seefeld, S.; Berry, N.; Gibbs, M.; Law, M., Colloidal iron pyrite(FeS₂) nanocrystal inks for thin-film photovoltaics. Journal of theAmerican Chemical Society 2011, 133 (4), 716-9; Gong, M. G.; Kirkeminde,A.; Ren, S. Q., Iron Pyrite (FeS₂) Broad Spectral and MagneticallyResponsive Photodetectors. DOI: 10.1002/adom.201200003, Advanced OpticalMaterials, 2012; Yuan, B.; Luan, W.; Tu, S. T., One-step synthesis ofcubic FeS₂ and flower-like FeSe₂ particles by a solvothermal reductionprocess. Dalton transactions 2012, 41 (3), 772-6.) These organic ligandsmay be exchanged for shorter molecules such as ethanedithiol (EDT),resulting in a chain of about ˜0.5 nm in length. Each of these organicligands may contribute to poor charge transfer and transport, electroniccoupling and electrical contact during operation of the devices. Anotherreason for the limited conversion efficiency is the degradation of FeS₂.For example, Tributsch et al. reported 2.8% efficiency for a FeS₂ singlecrystal-aqueous photoelectrochemical cell. (See Ennaoui, A.; Fiechter,S.; Smestad, G.; Tributsch, H. In Preparation of iron disulfide and itsuse for solar energy conversion, First World Renewable Energy Congress,1990: 1990; pp 458-464; Altermatt, P. P.; Kiesewetter, T.; Ellmer, K.;Tributsch, H., Specifying targets of future research in photovoltaicdevices containing pyrite (FeS₂) by numerical modelling. Solar EnergyMaterials & Solar cells 2002, 71, 181-195.) One of the main challengesin this device was the degradation of FeS₂ in iodide/triiodide aqueouselectrolyte. Furthermore, the instability of FeS₂ is also related to thepresence of surface states on FeS₂ which can trap the photoexcitedelectrons. Ultimately, whether FeS₂ is to be utilized in an energyharvesting or an energy storage device, the deficiencies of FeS₂ have tobe addressed and the devices are typically independently developed andoptimized, due to the challenges in integrating these two functions.

SUMMARY

Provided herein are FeS₂-based photovoltaic battery (PVB) devices andrelated methods.

In one aspect, photovoltaic battery devices are provided which comprisea transparent substrate; an active layer disposed over the transparentsubstrate, the active layer comprising a porous film of FeS₂nanocrystals and a halide ionic liquid infiltrating the porous film; andan electrode disposed over the active layer, wherein the device isconfigured such that under exposure to sunlight, photons incident on theactive layer are absorbed by the FeS₂ nanocrystals, generating a currentand a voltage, whereby a separation of charge within the active layer iscreated, which is discharged in the absence of the sunlight.

In another aspect, a methods of using a photovoltaic battery devices areprovided which comprise exposing the devices to sunlight, the devicescomprising a transparent substrate; an active layer disposed over thetransparent substrate, the active layer comprising a porous film of FeS₂nanocrystals and a halide ionic liquid infiltrating the porous film; andan electrode disposed over the active layer, wherein photons incident onthe active layer are absorbed by the FeS₂ nanocrystals, generating acurrent and a voltage, whereby a separation of charge within the activelayer is created, which is discharged in the absence of the sunlight.

Also provide are methods of making photovoltaic battery devices.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1 shows a cross-section of a schematic of a photovoltaic battery(PVB) device and a cross-section of a scanning electron microscope imageof a corresponding PVB device.

FIGS. 2A-C shows transmission electron microscope (TEM) images of FeS₂nanospheres (A), FeS₂ nanocubes (B) and a mixture of FeS₂ nanospheresand nanocubes (C).

FIG. 3 shows the power conversion efficiencies of FeS₂ PVB devicesmodified with ethanedithiol (EDT), 1-butyl-3-methylimidazolium bromide(BMII) and 1-hexyl-3-methylimidazolium bromide ([HMIM][Br]).

FIG. 4 shows the external quantum efficiency (EQE) spectrum of aBMII-treated FeS₂ NS/cube mixture PVB device.

FIG. 5 shows the differential reflection results measured from aBMII-treated FeS₂ NS/cube mixture PVB device (top) and a[HMIM][Br]-treated FeS₂ NS/cube mixture PVB device (bottom).

FIG. 6 shows the measured decay times for FeS₂ NS/cube mixture PVBdevices treated with either EDT, [HMIM][Br] or BMII. The inset shows thedifferential reflection results for the EDT-treated device.

FIG. 7A shows a schematic of the mechanism of a PVB device and FIG. 7Bshows a schematic of the energy band diagram of the device.

FIG. 8 shows the average specific capacitance values as a function ofscan rate for a BMII-treated FeS₂ NS/cube mixture PVB device.

FIG. 9 shows the capacity recycle measurement for a BMII-treated FeS₂NS/cube mixture PVB device.

FIG. 10 shows the cyclic voltammetry curves of FeS₂NS/cube mixture PVBdevice at scan rates of 0.1V/s, under room ambient light (as “dark”) and1100 nm NIR illumination (as “light”).

FIG. 11 shows the photoresponsivity (left boxes and left axis) anddetectivity (right boxes and right axis) of FeS₂ NS/cube mixture PVBdevices treated with either EDT, [HMIM][Br] or BMII.

FIG. 12A shows the current-time (I-t) characteristics of a FeS₂photocapacitor (PVB device) modified with [BMII] (bottom curve in leftportion of figure and upper curve in right portion of figure) and[Hmim][Tf₂N] (upper curve in left portion of figure and lower curve inright portion of figure). FIG. 12B shows the charge-discharge V-tcharacteristics of FeS₂ photocapacitor (PVB device), modified by BMII(upper curve) and [Hmim][Tf₂N] (lower curve).

DETAILED DESCRIPTION

Provided herein are FeS₂-based photovoltaic battery (PVB) devices andrelated methods.

The PVB devices are capable of achieving photoelectron conversion andenergy storage simultaneously under illumination (e.g., sunlight). Inthe dark, the PVB devices are capable of discharging the storedelectrical energy. The integration of these functions in a single deviceusing a single active layer comprising FeS₂ nanocrystals reduces thecomplexity, size and cost compared to convention energy harvesting andstorage solutions. In addition, a solution-based, room temperaturemethod for making the PVB devices is disclosed, which is amenable to lowcost, roll-by-roll production of FeS₂-based PVB devices on flexiblesubstrates. The PVB devices will find use in any application in which itis desirable to have a non-interruptive source of power, including insolar aircraft, solar vehicle and household electrical applications.

The PVB devices comprise a transparent substrate, an active layerdisposed over the transparent substrate and an electrode disposed overthe active layer. The active layer comprises a porous film of FeS₂nanocrystals and a halide ionic liquid infiltrating the porous film.

As further described below, the PVB devices may be configured such thatunder exposure to light (e.g., sunlight), photons incident on the activelayer are absorbed by the FeS₂ nanocrystals, generating a current and avoltage, whereby a separation of charge within the active layer iscreated, which is discharged in the absence of the light. Morespecifically, in the absence of light, the halide ionic liquid does notexperience much charge separation in the active layer, limited mostly tohalide anions being adsorbed onto the Fe²⁺-rich defect sites of the FeS₂nanocrystals. Under exposure to light, the p-type FeS₂ nanocrystals arephotoexcited, leading to hole generation and transport resulting inpartial positive charge on the FeS₂ nanocrystals. More halide anions arethen attracted to the FeS₂ surface by the columbic force, leaving behindthe cations of the halide ionic liquid, thus creating the separation ofcharge. This photocharging is due to the photoexcitation of the FeS₂nanocrystals. In the absence of the light, the double layer dissipatesand is discharged under the bias voltage. As also further describedbelow, the halide anions of the halide ionic liquid have a strongaffinity to the Fe-terminated cations of the surface of the FeS₂nanocrystals, thereby passivating the FeS₂ nanocrystals and enhancingelectronic coupling and suppressing photodegradation.

The active layer of the PVB devices comprises a porous film of FeS₂nanocrystals. FeS₂ nanocrystals having different shapes may be used. Forexample, the shape may be substantially spherical (such nanocrystals arereferred to herein as “nanospheres” or “NS”) or the shape may besubstantially cubical (such nanocrystals are referred to herein as“nanocubes” or “cubes” or “NC”). The largest dimension of the FeS₂nanocrystals is less than about 1000 nm. The dimension of the FeS₂nanocrystals may refer to the diameter (e.g., for nanospheres) or to thelength of a side (e.g., for nanocubes). The dimension may refer to anaverage dimension, by which it is meant an average value for apopulation of nanocrystals. Mixtures of FeS₂ nanocrystals havingdifferent shapes and sizes may be used. Suitable, non-limiting methodsfor making FeS₂ nanocrystals and porous films from the FeS₂ nanocrystalsare described in the Examples below. Regarding the disclosed methods formaking FeS₂ nanocrystals, experimental conditions may be adjusted (e.g.,reaction time and reaction temperature) to achieve FeS₂ nanocrystalshaving a particular desired shape, dimension and crystallinity.Regarding the disclosed methods for making the porous films of FeS₂nanocrystals, experimental conditions may be adjusted to achieve filmshave a particular desired characteristic (e.g., thickness, porosity,etc.). For example, the concentration of the FeS₂ nanocrystals and thevolume ratio of differently shaped/sized nanocrystals in a mixture maybe adjusted.

The active layer of the PVB devices further comprises a halide ionicliquid infiltrating the porous film of FeS₂ nanocrystals. The halideionic liquid is composed of halide anions and organic cations. Suitable,non-limiting halide anions include iodide (I⁻), bromide (Br⁻), chloride(Cl⁻), etc. Suitable, non-limiting organic cations include1-alkyl-3-methylimidazolium. Alkyl groups having different numbers ofcarbon atoms may be used. Suitable, non-limiting alkyl groups includeethyl, butyl, hexyl, etc. Other suitable halide ionic liquids includecetyltrimethylammonium halide (e.g., bromide),hexadecyltrimethylammonium halide (e.g., chloride), andtetrabutylammonium halide (e.g., iodide).

The PVB devices further comprise a transparent substrate. Suitable,non-limiting transparent substrates include glass coated with atransparent conducting film, e.g., indium tin oxide (ITO). Graphenecoated glass or flexible polymeric substrates are also suitablesubstrates. The PVB devices may further comprise a hole transport layerdisposed over the transparent substrate and underlying the active layer.Suitable, non-limiting hole transport layers include poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The PVB devicesfurther comprise an electrode. A variety of conductive materials, e.g.,metals may be used for the electrode. The PVB devices may comprise otherlayers and components typically used in photovoltaic and battery devicesfor optimal operation.

The PVB devices may be characterized by certain properties, includingpower conversion efficiency and specific capacity. The PVB device may becharacterized by a power conversion efficiency of at least about 2%, atleast about 4%, or at least about 6%. In some embodiments, the powerconversion efficiency is in the range of from about 3% to about 4%. Thepower conversion efficiency can be determined from current-voltagecurves measured under AM 1.5 illumination as described in the Examplesbelow. The PVB device may be characterized by a specific capacity of atleast about 35 mAhg⁻¹, at least about 40 mAhg⁻¹, at least about 50mAhg⁻¹, at least about 60 mAhg⁻¹, at least about 100 mAhg⁻¹, at leastabout 200 mAhg⁻¹ or at least about 300 mAhg⁻¹. The specific capacity canbe calculated from cyclic voltammogram curves measured at a specificscan rate (e.g., 0.1 V/s) under specific illumination conditions (e.g.,under dark) as further described in the Examples below.

An embodiment of a PVB device 100 is illustrated in FIG. 1. The PVBdevice 100 comprises a transparent substrate 102, a hole transport layer104 disposed on the transparent substrate, an active layer 106 disposedon the hole transport layer and an electrode 108 disposed on the activelayer. The active layer 106 comprises a porous film of FeS₂ nanocrystals110 a, 110 b and a halide ionic liquid 112 infiltrating the porous film.In this embodiment, the FeS₂ nanocrystals comprise a matrix of FeS₂nanospheres 110 a and FeS₂ nanocubes 110 b dispersed throughout thematrix.

The disclosed active layers may be used in the disclosed PVB devices orin photocapacitor devices. In some embodiments, the PVB devices may bereferred to as photocapacitors.

In some embodiments, the active layer and/or the PVB device issubstantially free of water and/or solvent (e.g., solvents used in thesynthesis and handling of the FeS₂ nanocrystals such as acetonitrile).In some embodiments, the active layer and/or the PVB device issubstantially free of ligands typically used to passivate FeS₂nanocrystals, e.g., octadecylamine, oleic acid, ethanedithiol, aromaticthiols, alkylamines, and mercaptocarboxylic acids, etc. In someembodiments, the active layer and/or the PVB device does not comprise asemiconductor material (e.g., a semiconductor layer or semiconductornanoparticles) having a majority carrier type (e.g., n-type) oppositethat of the FeS₂ nanocrystals. In some embodiments, the active layerconsists essentially of the porous film of FeS₂ nanocrystals and thehalide ionic liquid. The active layer may comprise a minor amount of thecomponents used in forming the active layer (e.g., using the methodsdescribed in the Examples below) and may still be considered to consistessentially of the porous film of FeS₂ nanocrystals and the halide ionicliquid.

In another aspect, a method for making a PVB device is provided whichcomprises depositing a porous film of FeS₂ nanocrystals on a transparentsubstrate (e.g., via a micro-centrifuge method) and infiltrating (e.g.,by spin-coating) the porous film with a halide ionic liquid to providean active layer. The substrate may be a transparent substrate on which ahole transporting layer has been previously deposited. If theas-deposited porous film of FeS₂ nanocrystals comprises any ligands orsurfactants associated with the FeS₂ nanocrystals during the depositionstep, the method may comprise removing the ligands and/or surfactantsprior to the infiltrating step. The method may further comprise dryingthe active layer (e.g., via exposure to heat). The method may furthercomprise depositing an electrode on the active layer.

In another aspect, methods of using the PVB devices are provided. Themethods involve exposing any of the disclosed PVB devices to light,wherein photons incident on the active layer are absorbed by the FeS₂nanocrystals, generating a current and a voltage, whereby a separationof charge within the active layer is created, which is discharged in theabsence of the light.

The disclosed PVB devices will be understood more readily by referenceto the following examples, which are provided by way of illustration andare not intended to be limiting.

EXAMPLES Materials and Methods

FeS₂ Nanocrystal Synthesis:

The FeS₂ nanocrystals were prepared using a modified protocol fromPuthussery et al., Colloidal Iron Pyrite (FeS₂) Nanocrystal Inks forThin-Film Photovoltaics, Journal of the American Chemical Society 2010,133 (4), 716-719. In detail, in one flask, 4 mmol sulfur solid particleswas dissolved in 5 mL diphenyl ether and sonicated until all sulfur wasdissolved and then degassed for one hour at 70° C. under argon. In aseparate vessel 0.5 mmol of FeCl₂ was dissolved in 12 g octadecylamine(ODA) and degassed for 1 hour at 120° C. to allow for decomposition. ForFeS₂ nanospheres (NS), the iron solution was then raised to 220° C. (forFeS₂ cubes the solution was kept at 120° C.) and the sulfur solution wasrapidly injected into the iron solution. The solution immediately turnedblack upon injection. This solution was allowed to react for 90 min.After the reaction was finished, the solution was allowed to cool to˜100° C. and halted with injection of methanol and crashed out usingcentrifugation. The FeS₂ nanocrystals were cleaned up using standardcrash out/wash method using chloroform/ethanol by centrifugation in a N₂glovebox. After cleaning the nanocrystals were redispersed in chloroformfor storage and characterization.

Materials Characterization:

All UV-Vis-NIR absorbance spectra were obtained on a UV-3600 ShimadzuSpectrophotometer. Room temperature x-ray powder patterns were obtainedusing monochromated Cu—Kα radiation (λ=1.54178 Å) on a Bruker ProteumDiffraction System equipped with Helios multilayer optics, an APEX IICCD detector and a Bruker MicroStar microfocus rotating anode x-raysource operating at 45 kV and 60 mA. The powders were mixed with a smallamount of Paratone N oil to form a paste that was then placed in a small(<0.5 mm.) nylon kryoloop and mounted on a goniometer head. TransmissionElectron Microscope (TEM) images were obtained using Field Emission FEITecnai F20 XT. Field Emission Scanning Electron Microscope (FESEM)images were obtained using LEO 1550 FESEM.

PVB Device Fabrication and Measurement:

The Schottky photovoltaic-battery devices are fabricated as follows: TheFeS₂ NS and cubes were dissolved in chloroform with concentration of 25mg/mL and 40 mg/mL, respectively. The mixture of FeS₂ NS (25 mg/mL) andcubes (40 mg/mL) was prepared with a volume ratio 2:1. Thepoly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)hole transport layer was spun-coated on the indium tin oxide (ITO)substrate at 3000 RPM for 1 min. The FeS₂ NS, FeS₂ cubes and mixturewere deposited on the ITO surface by the micro-centrifuge method at thespeed of 1500 RPM for 1 min. The FeS₂ nanocrystals were originallycapped with octadecylamine (ODA) surfactants, which were removed throughan ethanedithiol (EDT) treatment in acetonitrile solvent.

For the BMII-treated FeS₂ device, 20 μL of 1-butyl-3-methylimidazoliumiodide (BMII) was dropped onto the FeS₂ nanocrystal surface, then spincoated at 3000 RPM for 1 min and dried on a hot plate at 100° C. for 1min. The process was similar for the [HMIM][Br]-treated FeS₂ deviceusing 1-hexyl-3-methylimidazolium bromide (HMIM). This process allowsthe ionic liquid (IL) to seep into the porous FeS₂ active layer andachieves passivation of the nanocrystals by replacing long organicligands with halide anions bonded to cations. The infiltration ofsolution-cast materials into mesoporous structures has beeninvestigated. If the concentration of the solution is low enough, andthe solubility of the cast materials is high enough, the materials willpenetrate the pores as the solvent evaporates. Typically, the materialsform a “wetting” layer upon the internal surface of the mesoporous filmthat uniformly coats the pore walls throughout the whole thickness ofthe active layer. The degree of “pore-filing” can be controlled byvarying the solution concentration. If the concentration of the castingsolution is high, then maximum pore filing occurs and any “excess”materials form a “capping layer” on top of the filled mesoporous filmsurface. Since ILs do not evaporate quickly, there is good infiltrationof the IL into the pores of the active layer.

Finally, a patterned aluminum electrode (˜80 nm) was evaporated on thetop surface to complete the device. Current-Voltage (I-V) data weremeasured using a Keithley 2400 source-meter. The solar spectrum at AM1.5 was simulated to within class A specifications (less than 25%spectral mismatch) using a Xenon lamp and filter (Solar Light, Inc.) andintensity was adjusted to 100 mW/cm².

Results

The mineral iron pyrite (fool's gold) is an iron disulfide with theformula FeS₂. Pyrite's crystal structure is simple cubic much like NaCland can be thought of as iron atoms occupying the sodium position and S₂⁻² dumbells in place of the chlorine ion, which renders the Fe-dominant{100} surface. The Fe-terminated {100} surface facet dominant colloidalgrowth has been shown to result in the cubic structures. X-raydiffraction (XRD) patterns of the as-synthesized FeS₂ nanocrystals wereobtained and were consistent with the pyrite structure (JCPDS Card No1-079-0617), indicating that the nanocrystals are single-phase pyritewithout detectable marcasite, greigite, pyrrhotite, or other impurities.The XRD results show that after passivation, the FeS₂ nanocrystalsmaintain their crystalline state. Absorption spectra of FeS₂nanocrystals (nanosphere: NS, nanocube: cube, and their mixture) wereobtained which demonstrated promising light harvesting capabilitiesacross the visible to near-infrared (NIR) spectrum.

In this Example, three different ligands were utilized to examine theirpassivation effects on the performance of the PVB devices. The EDTligand was examined as a reference to compare with the halide basedionic liquid (IL) passivation effects. The two halide based ILs include1-hexyl-3-methylimidazolium bromide ([HMIM][Br]) and1-butyl-3-methylimidazolium iodide (BMII). After the halide atomicligand exchange, transmission electron microscopy (TEM) images wereobtained, which were used to investigate the stability of FeS₂nanocrystals without noticeable shape change. FIGS. 2A-C show thetransmission electron microscopy (TEM) images of well dispersed FeS₂ NS,cubes and mixture, respectively. The FeS₂ NS and cubes exhibited average13.4 nm diameter and a 47.5 nm side length, respectively. Highresolution TEM (HRTEM) images (inset of FIGS. 2A and 2B) show thelattice fringe of FeS₂ nanocrystals with the lattice spacing of 0.27 nm,matching the (200) plane of pyrite. Scanning TEM-EDS mapping images wereobtained which show the elemental mapping of the iodide passivated FeS₂nanocrystals which are embedded into the iodide ligand matrix. FIG. 1shows a schematic of the integrated FeS₂ PVB along with a cross-sectionSEM image of the device for comparison. It can be seen that the FeS₂ NSalso form a matrix around the FeS₂ cubes, which produces abulk-heterojunction structure which is backfilled by the spun-casthalide based IL to allow for efficient charge transfer and transport. A3D atomic force microscope (AFM) cross-section image of the FeS₂ NS-cubemixture PVB device was obtained, which further confirmed thewell-distributed FeS₂ nanocrystals stacked between two electrodes toform the PVB device.

Fourier transform infrared spectroscopy (FTIR) was used to confirm thatthe halide atomic ligand treatment of FeS₂ nanocrystals enables theremoval of surface insulating organic ligands. The FTIR spectra show thecomplete removal of the octadecylamine ligands (—CH₃ vibrations at 2874cm⁻¹ and 2959 cm⁻¹, —CH₂ vibration 1462 cm⁻¹ and 2935 cm⁻¹, and —NH₂vibration 3082 cm⁻¹ and 3144 cm⁻¹).

The ability of surface passivated FeS₂ nanocrystals to facilitate theextraction of photocurrent and photovoltage of PVB devices wasinvestigated. FIG. 3 summarizes the power conversion efficiencies amongthe FeS₂ NS, cube, and mixture based devices using different ligands(EDT, [HMIM][Br] and BMII). There is no photovoltaic performance of theEDT-treated PV devices (i.e., the power conversion efficiencies of thesedevices are about 0%). Using [HMIM][Br], the FeS₂ PVs achieve aphotoresponse and a low photovoltaic performance. In particular,current-voltage (J-V) curves measured under simulated AM 1.5 100 mW/cm²illumination were obtained. The FeS₂ NS device exhibited a short-circuitphotocurrent (J_(sc)) of 0.175 mA/cm², V_(oc) of 0.54 V with a fillfactor of 0.67 yielding an overall power conversion efficiency (η) of0.06%. When FeS₂ cubes were used, the device exhibited a lower circuitphotocurrent (J_(sc)) of 0.08 mA/cm², a higher V_(oc) of 0.59 V with afill factor of 0.52 yielding an overall power conversion efficiency (η)of 0.02%. The ITO/FeS₂ NS-cubes mixed/Al device exhibited ashort-circuit photocurrent (J_(sc)) of 0.17 mA/cm², a V_(oc) of 0.53 Vwith a fill factor of 0.58 yielding an overall power conversionefficiency (η) of 0.05%.

To investigate a more electro-negative halide atomic ligand effect, theiodide based BMII was used to passivate the FeS₂ NS/cube mixture whichachieved an efficiency of 4.07% under AM 1.5 illumination.Current-voltage (J-V) curves of BMII-treated FeS₂ devices measured underAM 1.5 (100 mW/cm²) illumination were obtained, showing that the FeS₂ NSdevice resulted in an average short-circuit current (J_(sc)) of 4.71mA/cm², V_(oc) of 0.39 V and a fill factor of 0.57, yielding an overallpower conversion efficiency (η) of 1.05%. The FeS₂ cube only deviceresulted in an average J_(sc) of 1.28 mA/cm², V_(oc) of 0.59 V and afill factor of 0.34, yielding an overall efficiency η of 0.25%. The mostefficient device was the FeS₂ NS-cube mixture which exhibited an averageJ_(sc) of 13.73 mA/cm², V_(oc) of 0.51 V and a fill factor of 0.59,yielding the maximum average efficiency η of 4.07%. These resultsconfirm the benefits of creating a bulk-heterojunction active layer withthe FeS₂ NS and cube mixture passivated by the iodide atomic ligands.The intimate contact between the FeS₂ NS matrix and cube componentaddresses the voids created by cube-only device, and also reduces theinterfacial area of a NS-only device. FIG. 4 shows the external quantumefficiency (EQE) spectrum of the BMII-treated FeS₂ NS/cube mixturedevice which exhibits spectral sensitivity spanning from the visible tothe NIR (400-1000 nm), matching very well with the UV-vis-NIR absorbancespectrum. The EQE shows maximum 80% photon-to-electron conversion in theblue spectral region, and a theoretical photocurrent density of 13.13mA/cm² is achieved by integrating the visible and NIR wavelength EQEspectrum.

It is known that FeS₂ materials have relatively scattered electricproperties due to the formation of surface defects related to sulfurvacancies and oxygen absorbance. These problems could be exacerbated inthe nanocrystal devices due to the high concentration of interfaces. Aninvestigation of the effect of halide atomic ligand passivation oncharge carrier dynamics, lifetime and related carrier density due wasundertaken. An optical ultrafast pump-probe technique was used tomeasure the carrier lifetime. A pump laser pulse of 100 fs and 750 nminjected charge carries by exciting electrons from the valence band tothe conduction band. These carriers were probed by a time-delay probepulse of 100 fs and 810 nm Reflection of the probe was collected andsent to a photodiode, which output was measured by a lock-in amplifier.By modulating the intensity of the pump beam with a mechanical chopper,the differential reflection could be measured, defined asΔR/R_(o)=(R−R_(o))/R_(o), where R and R_(o) are the reflections with andwithout the presence of the pump, respectively.

FIG. 5 shows the measured reflection signal as a function of the probedelay (the arriving time of the probe pulse at the sample with respectto the pump pulse), corresponding to FeS₂ NS/cube mixture PVB devicestreated with BMII or [HMIM][Br]. The measured decay time can beattributed to the carrier lifetime of FeS₂ nanocrystals. The signaldecays exponentially, so by fitting the curve, the carrier lifetime ofthe passivated FeS₂ nanocrystals can be obtained. As summarized in FIG.6, the EDT, [HMIM][Br] and [BMII] treated PVB devices exhibit a carrierlifetime of 8 ps, 126 ps and 189 ps, respectively. The ultrashortlifetime of the EDT-treated PVB device is consistent with the lowefficiency measured. The halide ligand-passivated devices have longercarrier lifetimes and the higher efficiency may be attributed to thepassivation of FeS₂ surface defect states and fast carrier transport.

The mechanism of the hybrid PVB device is illustrated in FIGS. 7A-B.Under dark, the power generation process is based on the FeS₂ batterydischarging process. The IL-only device without FeS₂ nanocrystals doesnot exhibit an ability to generate power, which confirms thecontribution of the FeS₂ nanocrystals. The potential difference orvoltage of battery under dark is interpreted using the capacitor model.The iodide (I⁻) anions are physically adsorbed onto the Fe-rich surfaceof the nanocrystal facets by electrostatic adsorption, and the redoxprocess occurs on the cathode side to accumulate electrons. The BMI⁺cations of the IL move to the other direction toward the PEDOT:PSS/ITOanode side. The accumulated charges at two electrodes can provide apotential difference to drive the flow of electrons through an externalload. The separated charges on the opposite electrodes accelerate thecharge transport process. In the illuminated state, there are twoprocesses present in this device. The main scheme is similar to thetraditional Schottky PV device, where the passivated FeS₂ nanocrystalsabsorb photons and produce photocurrent and photovoltage, whichdominates the PV performance. As shown in FIG. 7B, in the Schottky PVdevice, band bending occurs at the junction between the FeS₂ and Alelectrode, causing photoelectrons generated in the FeS₂ to be injectedinto the Al electrode. The other process observed is the chargingprocess of the pyrite battery. The charging process includes the redoxmediator regeneration (I⁻) from the photoexcited electrons of FeS₂ underillumination.

Subsequently the electrons were injected into Al cathode to complete thecircuit. Cyclic voltammogram (CV) curves of BMII-treated FeS₂ NS-cubemixture PVB devices were obtained. The specific capacity (C_(s)) wascalculated from the CV loops using Cs=∫i/m dt, where i is the oxidationor reduction current, dt is the time differential, and m is the mass ofthe active electrode materials. As shown in FIG. 8, the BMII-treatedFeS₂ NS/cubes mixture PVB device showed a specific capacity of 57.8mAhg⁻¹ and 30.0 mAhg⁻¹ (based on the mass of FeS₂) at a scan rate of 50mVs⁻¹ and 300 mVs⁻¹ (corresponding to 10 s of a full charging ordischarging period), respectively, suggesting excellent rate capabilityof the FeS₂ material. As shown in FIG. 9, the capacity recyclemeasurement shows FeS₂ PVB device stability even after 100 cycles, whichis different from a traditional Li-ion FeS₂ battery.

FIG. 10 shows the cyclic voltammetry curves of FeS₂NS/cube mixture PVBdevice at scan rates of 0.1V/s, under room ambient light (as “dark”) and1100 nm NIR illumination (as “light”). The I-V curves ofITO/PEDOT/BMII/Al device (no FeS₂ nanocrystals) were obtained under darkand AM-1.5 illumination. Both of the curves went through the (0,0)point, indicating that the ionic liquid BMII alone could not producepotential difference between the two electrodes and provide power forthe external load.

In summary, the solution processed FeS₂ nanocrystal schottkyphotovoltaic-battery device has been demonstrated as non-interruptedpower source. The FeS₂ nanocrystal PVB devices exhibit 4.07% powerconversion efficiency under AM 1.5 illumination and 57.8 mAhg⁻¹ specificcapacity under dark. The halide atomic ligand passivation strategy fromthe ionic liquids enables high carrier mobility and excellent devicestability, whilst using low cost chemicals readily available and easilyprocessed. The iodide based ligand passivation of FeS₂ nanocrystalsshows 20 times increase of carrier lifetime, in comparison with the EDTligand exchange process.

Additional Results

PVB devices were illuminated using 1100 nm near-infrared (NIR) light.The NIR light source used here could be used to photoexcite FeS₂ only.As shown in FIG. 11, the high performance BMII-passivated FeS₂ PVBdevices exhibited a photoresponse of 37.6 A/W (left box and left axis)and photodetectivity of 1.6×10¹¹ (right box and right axis) under 1100nm illumination. With the aim of testing FeS₂ photocharging ability,electrochemical capacitance measurements under 1100 nm NIR light andcyclic voltammogram measurements of BMII-passivated FeS₂ PVB deviceswere obtained at a scan rate of 0.1 V/s. A specific capacity (C_(s)) of37.5 mA·h/g under room ambient light (as “dark”) and 46.0 mA·h/g underthe 1100 nm NIR illumination (as “light”) were obtained. The specificcapacity increases by 23% under the NIR light illumination, whichconfirms the photocharging from NIR photoresponsive FeS₂ nanocrystals.The FeS₂ PVB devices were also tested under different scan rates (0.2,0.3, 0.4 and 0.5 V/s). The CV curves remained similar in shape for thedifferent scan rates indicating a high electrochemical stability andcapacitance.

The photocurrent of EDT modified FeS₂ NS, cube, and mixture devices wasmeasured. Cube-only pyrite active material exhibited the lowestphotocurrent. When replacing cubes with NS, the photocurrent increasessignificantly. By mixing the FeS₂ NS and cube together, the photocurrentreaches its maximum. Thus, cubes are more active than the quantum dots,but benefit from a matrix surrounding them to help with charge transportdue to their large size and poor stacking in films. Knowing that themixture of both shapes gives the best performance, different ILs weretested to compare the effect of different halide ions on thephotocurrent generation. A higher photocurrent is observed by using the[Hmim][Br] as compared to EDT based passivation. When replacing the Br⁻ion with the I⁻ ion in the BMII ionic liquid, the current again takes aneven more substantial enhancement. The increase in the photocurrentcould be related to the passivation of the pyrite surface defects, wherethe I⁻ ions show better effective passivation than that of Br⁻.

To understand the size effect of different ions on the transportproperties of FeS₂ photocapacitors, an ionic liquid with a bigger anion([Tf₂N]⁻) was selected. As shown in FIGS. 12A-B, the current-time andthe voltage-time characteristics of the FeS₂ photocapacitor modifiedwith [BMII] and [Hmim][Tf₂N] were obtained. The rate of discharging isseen to change between the two electrolytes, where I⁻ charges fasterthan the bigger [Tf₂N] due to mobility's proportionality to the radiusof ions (v≈1/r²). When the small sized I⁻ is used, it can passivate FeS₂nanocrystals more effectively and move faster, which allows it toexhibit higher capacity and fast discharging rate. The photon-inducedenergy stored in the capacitor can be calculated by integrating I-tcurves (current intensity reaches to the saturation under AM-1.5 lightillumination). According to W_(stored)=QU=U∫∫dtdt, where W_(stored), Q,U, i and t are storage energy, electric quantity, applied voltage,current and time, the photocharged energy density was calculated as1.13×10⁴ J/g for the BMII-based PVB device and 0.3×10⁴ J/g for the[Hmim][Tf₂N]-based PVB device. The former is nearly 4 times than that ofthe latter case, which further confirms the ionic size effect on thetransport and capacitance of the PVB devices. This can be partiallyattributed to the steric effect of the larger size of the [Tf₂N] ions,reducing the energy density from the electric double layer capacitors.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”. Still further, the use of “and” or “or” is intended to include“and/or” unless specifically indicated otherwise.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art, all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeincludes the number recited and refers to ranges which can besubsequently broken down into subranges as discussed above. Finally, aswill be understood by one skilled in the art, a range includes eachindividual member.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A photovoltaic battery device comprising: atransparent substrate; an active layer disposed over the transparentsubstrate, the active layer comprising a porous film of FeS₂nanocrystals and a halide ionic liquid infiltrating the porous film; andan electrode disposed over the active layer, wherein the device isconfigured such that under exposure to sunlight, photons incident on theactive layer are absorbed by the FeS₂ nanocrystals, generating a currentand a voltage, whereby a separation of charge within the halide ionicliquid of the active layer is created, which is discharged in theabsence of the sunlight.
 2. The device of claim 1, wherein the FeS₂nanocrystals comprise FeS₂ nanospheres, FeS₂ nanocubes, or combinationsthereof.
 3. The device of claim 1, wherein the FeS₂ nanocrystalscomprise a matrix of FeS₂ nanospheres and FeS₂ nanocubes dispersedthroughout the matrix.
 4. The device of claim 1, wherein the halideionic liquid is a 1-alkyl-methylimidazolium halide.
 5. The device ofclaim 4, wherein the alkyl group is butyl or hexyl and the halide isiodide or bromide.
 6. The device of claim 1 characterized by a powerconversion efficiency of at least about 4% under AM 1.5 illumination anda specific capacity of at least about 35 mAhg⁻¹ in the dark.
 7. Thedevice of claim 1, wherein the active layer is substantially free ofwater and/or solvent.
 8. The device of claim 1, wherein the active layeris substantially free of octadecylamine, oleic acid, ethanedithiol, anaromatic thiol, an alkylamine, and/or a mercaptocarboxylic acid.
 9. Thedevice of claim 1, wherein the device does not comprise a semiconductormaterial having a majority carrier type opposite that of the FeS₂nanocrystals.
 10. A photovoltaic battery device comprising: atransparent substrate; an active layer disposed over the transparentsubstrate, the active layer comprising a porous film of FeS₂nanocrystals and a halide ionic liquid infiltrating the porous film; andan electrode disposed over the active layer, wherein the device isconfigured such that under exposure to sunlight, photons incident on theactive layer are absorbed by the FeS₂ nanocrystals, generating a currentand a voltage, whereby a separation of charge within the active layer iscreated, which is discharged in the absence of the sunlight, wherein theactive layer consists essentially of the porous film of FeS₂nanocrystals and the halide ionic liquid.
 11. A photovoltaic batterydevice comprising: a transparent substrate; a hole transport layerdisposed on the transparent substrate; an active layer disposed on thehole transport layer, the active layer comprising a porous film of FeS₂nanospheres, FeS₂ nanocubes, or combinations thereof, and a1-alkyl-methylimidazolium halide ionic liquid infiltrating the porousfilm; and an electrode disposed on the active layer, wherein the deviceis configured such that under exposure to sunlight, photons incident onthe active layer are absorbed by the FeS₂ nanospheres, FeS₂ nanocubes,or combinations thereof, generating a current and a voltage, whereby aseparation of charge within the halide ionic liquid of the active layeris created, which is discharged in the absence of the sunlight.
 12. Thedevice of claim 11, wherein the FeS₂ nanocrystals comprise a matrix ofFeS₂ nanospheres and FeS₂ nanocubes dispersed throughout the matrix. 13.The device of claim 11, wherein the alkyl group is butyl or hexyl andthe halide is iodide or bromide.
 14. A method of using a photovoltaicbattery device, the method comprising exposing the device to sunlight,the device comprising: a transparent substrate; an active layer disposedover the transparent substrate, the active layer comprising a porousfilm of FeS₂ nanocrystals and a halide ionic liquid infiltrating theporous film; and an electrode disposed over the active layer, whereinphotons incident on the active layer are absorbed by the FeS₂nanocrystals, generating a current and a voltage, whereby a separationof charge within the halide ionic liquid of the active layer is created,which is discharged in the absence of the sunlight.
 15. The method ofclaim 14, wherein the device further comprises a hole transport layerdisposed on the transparent substrate, the active layer is disposed onthe hole transport layer and the electrode is disposed on the activelayer, and further wherein the FeS₂ nanocrystals comprise FeS₂nanospheres, FeS₂ nanocubes, or combinations thereof, and the halideionic liquid is a 1-alkyl-methylimidazolium halide.